Inorg. Chem. 1995,34, 4600-4604
4600
Strong Metal-Metal Coupling in a Dinuclear (Terpyridine)(bipyridine)ruthenium Mixed-Valence Complex Incorporating the Bridging Ligand 1,4=Dicyanamidobenzene Dianion Ali R. Rezvani, Christopher E. B. Evans, and Robert J. Crutchley* The Ottawa-Carleton Chemistry Institute, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 Received June 14, I994@
The complex, [{Ru(terpy)(bpy)}2@-dicyd)][PF&, where terpy = 2,2’,2’’-terpyridine, bpy = 2,2’-bipyridine, and dicyd2- = 1,4-dicyanamidobenzenedianion, has been synthesized and characterized by cyclic voltammetry and spectroelectrochemicalmethods. A quantitative absorption spectrum of the radical anion dicyd’- has also been determined. The mixed valence ion, [{Ru(terpy)(bpy)}2@-dicyd)13+ is strongly coupled with Kc = 2.7 x lo7 and has an intervalence band at iZ = 1090 nm (emax= 3000 M-I cm-I, V112 = 1800 cm-I). The mixed-valence properties of this complex were compared to its ammine analogue [{(NH3)5Ru}2@-dicyd)13+ and rationalized by the perturbation of spectator ligands on the interaction of ruthenium ions with the dicyd2- superexchange pathway. The dependence of intervalence oscillator strength on the nature of the mixed-valence complex was also discussed.
Introduction The mechanism for metal-metal coupling in dinuclear ruthenium complexes which incorporate the 1,4-dicyanamidobenzene dianion (dicyd2-) bridging ligand,
is dominated by superexchange via the n HOMO of dicyd2-. The interaction of ruthenium nd orbitals with the n HOMO of dicyd2- (shown schematically
where the size of the atomic orbital approximates its contribution to the molecular orbital),’ creates a superexchange pathway which is both symmetry and energy favorable. In recent studies?~~ we have shown that the comproportion constants of the mixed valence complexes, [{(NH3)5Ru}2@-L)13+,where L = dicyd2- and 1,ddicyanamido-2,5-dimethylbenzenedianion (Me2dicyd2-)), are remarkably sensitive to the donor properties of the solvent. This solvent dependence classifies these complexes as valence localized class I1 mixed-valence systems4 even though the magnitude of their comproportionation constants in poor donor solvents approaches that of the valence Abstract published in Advance ACS Abstracts, July 15, 1995. (1) Aquino, M. A. S.; Lee, F. L.; Gabe, E. J.; Bensimon, C.; Greedan, J. E.; Crutchley, R. J. J. Am. Chem. Soc. 1992, 114, 5130. (2) Naklicki, M. L.; Crutchley, R. J. J. Am. Chem. Soc. 1994, 116, 6045. (3) Naklicki, M. L.; Crutchley, R. J. Inorg. Chim. Acta 1994, 225, 123. (4) (a) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (b) Crutchley, R. J. Adv. Inorg. Chem. 1994, 41, 273.
delocalized class I11 Creutz-Taube ion, [{( N H ~ ) ~ R u } ~ @ pyrazine)]S+.s In this study, we have synthesized the novel dinuclear [PF&. Cyclic voltamcomplex, [{Ru(terpy)(bpy)}2@-dicyd)] metry and spectroelectrochemical methods showed that the complex, [{Ru(terpy)(bpy)}2@-dicyd)13+, is a strongly coupled class I11 mixed-valence ion. The properties of this complex are compared to its ammine analogue, [{(NH3)5Ru}2@-dicyd)13+,and rationalized in terms of the electronic perturbations that are introduced when an ammine ligand is replaced by pyridine.
Experimental Section Physical Measurements. The equipment used to perform cyclic voltammetry, IR, ‘H-NMR, and UV-vis-near-IR spectroscopy has been described in a previous paper.6 Spectroelectrochemistry was performed with a Pyrex-quartz cell of published design? on acetonitrile solutions containing 1.O x 10-5 M [{Ru(terpy)(bpy)}2@-dicyd)l[PF6]2 and 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) electrolyte. Platinum-mesh working, platinum-wire counter and silver-wire reference electrodes were used. The solutions were degassed and agitared by bubbling argon through a Teflon needle. The potential at the working electrode was controlled by using a BAS CV-27 apparatus. Aldrich anhydrous acetonitrile and Anachemia accusolv grade dimethylformamide (DMF) were used as received. TBAH was recrystallized twice from ethanol and vacuum dried at 110 “C overnight. TBAB, tetrabutylammonium tetraphenylborate was synthesized by combining aqueous solutions of NaBPh4 and tetrabutylammonium bromide. TBAB was separated, washed with water and vacuum dried at 110 “C overnight. Ferrocene (E” = 665 mV vs NHE)8 was used as an internal reference. Elemental analysis was performed by Canadian Microanalytical Services Ltd. Reagents. All chemicals and solvents were reagent grade or better and used as received. 2,2’,2”-Terpyridine (terpy) and 2,2‘-bipyridine (bpy) were purchased from Aldrich. 1,4-Dicyanamidebenzene (di-
@
( 5 ) Creutz, C.; Chou, M. H.Inorg. Chem. 1987, 26, 2995. (6) Rezvani, A. R.; Crutchley, R. J. Inorg. Chem. 1994, 33, 170.
(7) Brewer, K. J.; Calvin, M.; Lummpkin, R. S.; Otvos, J. W.; Spreer, L. 0. Inorg. Chem. 1989, 28,4446. (8) Gennett, T.; Milner, D. F.; Weaver, M. J. J. Phys. Chem. 1985, 89, 2787.
0020- 1669/95/1334-4600$09.00/0 0 1995 American Chemical Society
Strong Metal-Metal Coupling cydHz),] [AsPh&[dicyd],' Ru(terpy)Cl3? and [R~(bpy)(terpy)Ci][PF6]'~ were prepared by following literature procedures. Preparation of [{Ru(bpy)(terpy)}2@-dicyd)][PF&.DMF. A mixture of [[Ru(bpy)(terpy)C1][PF6](224 mg, 0.33 mmol) and AgPF6 (0.85 mg, 0.33 mmol) was placed in acetone (50 mL) and stirred at a reflux for 3 h. The reaction mixture was then filtered and to the filtrate was added dicydH2 (27 mg, 0.17 mmol). The solution was vacuum degassed and then stirred with slight heating (25-30 "C) under argon overnight. The solvent was removed and the crude product purified by column chromatography by using grade V alumina type WA-1 (Sigma) and a 2:1 mixture of CH$.~/CHICN as eluent. Three major bands were eluted in the order: a yellow band (possibly deprotonated ligand), a purple band (a mononuclear Ru(I1)-terpy derivative), and finally a brown band of the dinuclear complex. The product eluent was evaporated to dryness and the product recrystallized by the diffusion of ether into a dimethylfonnamide solution of the complex. Darkbrown fine crystals of the dinuclear complex were collected and washed with ether and vacuum dried. Yield: 51 mg, 21%. Anal. Calcd for C ~ I H ~ ~ N & F I & C, . I ?48.83 : (ppm); H, 3.27; N, 14.01. Found: C, 48.08; H, 3.59; N, 14.33. 'H NMR in dimethyl sulfoxide-&, relative to TMS at 0.00 ppm: DMF resonances at 2.74 (3 H, singlet), 2.91 (3 H, singlet) and 7.95 ( 1 H, singlet); phenyl protons at 5.50 (4 H, singlet); terpy and bpy protons at 7.13 (2 H, triplet), 7.39 (2 H, doublet), 7.41 (4 H, triplet), 7.65 (4 H, doublet), 7.82 (2 H, triplet), 8.01 (4 H, triplet), 8.10 ( 2 H. triplet), 8.24 (2 H, triplet), 8.40 (2 H, triplet), 8.67 (4 H, doublet), 8.70 (2 H, doublet), 8.81 (4 H, doublet), 8.94 (2 H, doublet), 9.65 (2 H, doublet).
Results The crude product of the reaction of [Ru(terpy)(bpy)Cl]+ with neutral dicydH2 is suggested to be the dinuclear complex [{Ru(terpy)(bpy)}2@-dicydH2)I4+, based on the observation of a protonated cyanamide v(NCN) band at 2250 cm-' in the IR spectrum of the crude product obtained by reducing the reaction solution to dryness." It seems likely that the dicydH2 is actually bound to Ru(I1) at this stage because the bromide salt of the crude complex (precipitated by the addition of tetrabutylammonium bromide to the reaction solution) still possesses a protonated cyanamide v(NCN) band at 2225 cm-I. Deprotonation of the bridging ligand apparently occurred during purification by chromatography using grade V alumina as shown by the characteristic anionic cyanamide v(NCN) band observed at 2151 cm-' and the stoichiometry required by the elemental analysis for the final product [ {R~(terpy)(bpy))2@-dicyd)]~+. The deprotonation of the bridging ligand was not pursued further in this study. The cyclic voltammograms of the mononuclear complex [Ru(terpy)(bpy)Cl]+ and the dinuclear complex [{Ru(terpy)(bpy)}z@-dicyd)12+in DMF solutions are compared in Figure 1. The cyanamide anion group is a pseudo halogen and is expected to perturb the Ru(IIVI1) couple in much the same way as a chloride anion ligand. Thus, for the dinuclear complex, Figure lB, the Ru(III/II) couples occur in approximately the same region as the Ru(III/II) couple of [Ru(terpy)(bpy)Cl]+ at 1.025 V vs NHE (Figure 1A) but are split into two one-electron waves (at 0.250 and 0.690 V vs NHE) because of significant metal-metal coupling. The Ru(IIVI1) reduction couples for both complexes appear quasi-reversible with an average separation between cathodic and anodic waves of 70 mV that is largely independent of scan rate between 50-250 mV/s. From the difference in Ru(IIM1) couples of the dinuclear complex (AE = 440 mV), ( 9 ) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
1404. (10) (a) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151. (b) Takeuchi, K. J.; Thompson, M. S.; Pipes, D. W.; Meyer, T. J. Inorg. Chem. 1984, 23, 1845. (11) Naklicki. M.L.: Crutchley, R. J. Inorg. Chem. 1989, 28, 4226.
Inorganic Chemistry, Vol. 34, No. 18, 1995 4601
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Figure 1. Cyclic voltammograms: (A) [Ru(bpy)(terpy)ci][PF6]; (B) [{Ru(terpy)(bpy)}&~-dicyd)][PF6]2 in dimethylformamide.
the comproportionation constant ( K c )for the formation of the Ru(III/II) mixed-valence complex was determined to be 2.7 x lo7. This comproportionation constant is approximately the same as that found for the Creutz-Taube ion [{(NH&Ru)2@-pyrazine)15+(Kc = 8.54 x lo6 in DMF)5 and by analogy, is supportive of a class I11 assignment for the mixed-valence complex of this study.4 The PF6- salt of the dinuclear complex proved sparingly soluble in all but strong donor solvents and this severely limited studies of solvent-dependent properties. However, the tetraphenylborate salt of the complex was sufficiently soluble in acetonitrile to permit a cyclic voltammetry study. At a scan rate of 100 mV/s and with 0.025 M TBAB electrolyte, two Ru(IIVI1) couples were observed at 0.415 and 0.840 V vs NHE which is roughly the same separation between Ru(III/II) couples (AE) that is observed in DMF. In contrast, [{(NH3)5Ru}2@-Me2dicyd)14+,shows dramatic solvent perturbation of metalmetal coupling where in nitromethane AE = 368 mV, while in DMSO AE = 198 mV.' The solvent dependence of these Ru(III/II) couples was suggested to be caused by the donoracceptor interaction between the solvent molecules and the ammine ligands. Importantly, this effect is absent in [{Ru( t e ~)@PY y 112@-dic~ d)I2+. Three one electron ligand reduction waves are observed between - 1.O and -2.0 V, in the voltammogram of [Ru(terpy)(bpy)Cl]', Figure 1A. The first reduction wave probably corresponds to the reduction of the coordinated terpy ligand because of the greater stability of terpy's x* orbital, compared to that of bpyloa.l2 [Ru"(terpy)(bpy)Cl]+
+ e- ==[Ru"(terpy*-)(bpy)Cl]
(1)
The above couple occurs at a potential of - 1.100 V vs NHE with cathodic and anodic peak separation (70 mV) independent of scan rate from 50 to 250 mV/s. This is followed closely by the irreversible reduction of the coordinated bpy ligand [Ru"(terpy'-)(bpy)Cl]
+ e- - [Ru"(terpy'-)(bpy'-)Cl](2)
The cathodic peak maximum of this couple occurred at - 1.320 V vs NHE and a weak anodic wave that is associate with this couple is seen at approximately -0.9 V vs NHE. This anodic wave does not appear unless the potential is swept past - 1.320 (12) Berger, R. M.: McMillin. D. R. Inorg. Chem. 1988, 27. 4245.
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